PostFix and or Courier IMAP question

The ubuntu how-to was very straight forward. Up and running in short order.

I would like to set up default Outbox, Sent Items and Draft directories. We will be using either Thunderbird or Evolution and maybe down the road a web interface such as squirrel mail. We will only be using IMAP.

You can either have your users create those folders with their email client, or you create those folders with ISPConfig when you create a new user.
For the latter, you'd have to modify the function user_insert() in /root/ispconfig/scripts/lib/config.lib.php.